Internal combustion engines have a cylinder block and at least one cylinder head which are connected to one another to form the cylinders and the combustion chambers thereof. The cylinder block, as the upper crankcase half, generally serves for the mounting of the crankshaft and for accommodating the piston and the cylinder sleeve of each cylinder. The piston may also, with the omission of a sleeve as an intermediate element, be mounted and guided directly in a bore of the block. In the context of the present disclosure, both the cylinder sleeve and the bore are subsumed under the expression “cylinder barrel”.
The cylinder head normally serves for accommodating the valve drives required for the charge exchange. During the course of the charge exchange, the discharge of the combustion gases via the exhaust-gas discharge system takes place via the at least one outlet opening, and the feed of the combustion air via the intake system takes place via the at least one inlet opening of the cylinder. In the case of four-stroke engines, use is typically made almost exclusively of lifting valves for the control of the charge exchange. The valve, including the associated actuating mechanism, is referred to as valve drive.
The crankshaft which is mounted in the crankcase absorbs the connecting rod forces and transforms the oscillating stroke movement of the pistons into a rotational movement of the crankshaft. The upper crankcase half formed by the cylinder block is regularly supplemented by the oil pan, which can be mounted on the cylinder block and which serves as the lower crankcase half.
In the development of internal combustion engines, it is constantly sought to minimize fuel consumption and reduce pollutant emissions.
Fuel consumption poses a problem in particular in the case of Otto-cycle engines, that is to say in the case of spark-ignition internal combustion engines. The reason for this lies in the principle of the operating process of the traditional Otto-cycle engine. The traditional Otto-cycle engine operates with external mixture formation and a homogeneous fuel-air mixture, in which the desired power is set by varying the charge of the combustion chamber, that is to say by quantity regulation. By adjusting a throttle flap which is provided in the intake system, the pressure of the inducted air downstream of the throttle flap can be reduced to a greater or lesser extent. For a constant combustion chamber volume, it is possible in this way for the air mass, that is to say the quantity, to be set by the pressure of the inducted air. This also explains why quantity regulation has proven to be disadvantageous specifically in part-load operation, because low loads require a high degree of throttling and a large pressure reduction in the intake system, as a result of which the charge exchange losses increase with decreasing load and increasing throttling.
One approach for dethrottling the Otto-cycle working process is to utilize direct fuel injection. The injection of the fuel directly into the combustion chamber of the cylinder is considered to be a suitable measure for noticeably reducing fuel consumption even in Otto-cycle engines. The dethrottling of the internal combustion engine is realized by virtue of quality regulation being used within certain limits.
With the direct injection of the fuel into the combustion chamber, it is possible in particular to realize a stratified combustion chamber charge, which can contribute significantly to the dethrottling of the Otto-cycle working process because the internal combustion engine can be leaned to a great extent by means of the stratified charge operation, which offers thermodynamic advantages in particular in part-load operation, that is to say in the lower and middle load range, when only small amounts of fuel are to be injected.
Direct injection is characterized by an inhomogeneous combustion chamber charge which is not characterized by a uniform air ratio but which generally has both lean (λ>1) mixture parts and rich (λ<1) mixture parts. The inhomogeneity of the fuel-air mixture is also a reason why the particle emissions known from the diesel engine process are likewise of relevance in the case of the direct-injection Otto-cycle engine, whereas said emissions are of almost no significance in the case of the traditional Otto-cycle engine.
There is relatively little time available for the injection of the fuel, for the mixture preparation in the combustion chamber, specifically the mixing of air and fuel and the preparation of the fuel within the context of preliminary reactions including evaporation, and for the ignition of the prepared mixture.
The resulting demands placed on the mixture formation relate not only to the direct-injection Otto-cycle engine but basically to any direct-injection internal combustion engine, and thus also to direct-injection diesel engines.
Since, in the case of direct injection, there is only little time available for the mixture formation, there is a need for measures with which the mixture formation is assisted and accelerated in order to substantially homogenize the fuel-air mixture before the ignition, at least as long as there is no demand for stratified-charge operation. In this context, the distribution of the fuel in the combustion chamber, and thus also the injection of the fuel, are of particular importance.
In the case of the direct-injection Otto-cycle engine, a distinction can be made between substantially three methods for mixture formation.
In the case of the air-controlled method, it is sought to influence the mixture formation by a movement forcibly imparted by the inlet flow as the air is inducted into the combustion chamber. It is sought in this way to achieve a good mixture of the inducted air with the injected fuel, wherein it is the intention for a direct impingement of the injected fuel on the internal walls of the combustion chamber to be prevented by the generated charge movement or flow.
For example, the generation of a so-called tumble or swirling flow can accelerate and assist the mixture formation. A tumble is an air vortex about an imaginary axis which runs parallel to the longitudinal axis, that is to say to the axis of rotation, of the crankshaft, by contrast to a swirl, which constitutes an air vortex whose axis runs parallel to the piston longitudinal axis, that is to say the cylinder longitudinal axis.
The arrangement and the geometry of the intake system, that is to say of the intake lines, have a significant influence on the charge movement and thus on the mixture formation, wherein the charge movement in the cylinder is concomitantly influenced by the combustion chamber geometry, in particular by the geometry of the piston crown or of a piston depression that is optionally provided in the piston crown. In the case of direct-injection internal combustion engines, use may be made of depressions that are rotationally symmetrical to the piston longitudinal axis, in particular omega-shaped depressions. Owing to the constricted space conditions in the cylinder head, an optimization of the intake lines with regard to mixture formation and charge exchange may not be possible, or may not be fully possible, or may be possible only if disadvantages are accepted elsewhere.
In the case of the wall-controlled method, the fuel is injected into the combustion chamber in such a way that the injection jet is purposely directed toward a wall delimiting the combustion chamber, preferably into a depression provided on the piston crown. Here, it is the intention for the fuel jet to be, as a result of the impingement, broken up into multiple jet parts and diverted such that as large an area of the combustion chamber as possible is encompassed by the fuel jets. In particular, some of the injected fuel must be diverted into the vicinity of the ignition device in order to form an ignitable fuel-air mixture there with the inducted air.
Whereas it is the case in the air-controlled method that a direct impingement of the injected fuel on the combustion chamber internal walls should be prevented, this is desired in the case of the wall-controlled method. The wetting of the combustion chamber internal walls with fuel promotes oil thinning, and disadvantageously increases the untreated emissions of unburned hydrocarbons and the particle emissions.
In the case of the jet-controlled method, the fuel is injected in targeted fashion in the direction of the ignition device, which is achieved by a corresponding orientation of the injection jets and a correspondingly coordinated arrangement of the injection device and ignition device, for example by an arrangement of both the ignition device and also the injection device centrally in the cylinder head on the side facing the piston crown.
The fuel is transported and distributed substantially owing to the impetus of the injection jets, such that the mixture formation is relatively independent of the combustion chamber geometry, which constitutes a significant advantage in relation to the two other methods. The jet-controlled method is suitable in particular for stratified-charge operation of the internal combustion engine, because firstly, an ignitable mixture can be formed in a closely confined area around the ignition device, and secondly, a low fuel concentration can be realized in large areas of the combustion chamber.
Most methods for mixture formation exhibit both an air-controlled component and a jet-controlled component.
In some examples, the injection device is arranged in the cylinder head on the side facing the piston crown. Depending on the penetration depth of the injection jets, the injected fuel quantity and the injection time, that is to say the position of the piston, a greater or lesser fraction of the fuel impinges on the combustion chamber internal walls during the injection, in particular on the piston crown and the cylinder barrel, and mixes with the oil film adhering thereto. The fuel passes together with the oil into the crankcase and thus contributes significantly to oil thinning. The wetting of the combustion chamber internal walls with fuel furthermore has an adverse effect on the untreated emissions of unburned hydrocarbons and on the particle emissions.
Use may therefore also be made of injection devices whose injection jets exhibit a reduced or small penetration depth into the combustion chamber. In practice, it has however been found that, despite a reduced penetration depth, the combustion chamber internal walls are wetted with fuel, specifically even if the injection jets do not directly strike the combustion chamber internal walls. The cause of this is non-evaporated liquid fuel which, in the form of fuel droplets, is transported to the combustion chamber internal walls, and wets these, owing to the charge movement in the combustion chamber.
In some examples, the cylinders of the internal combustion engine are each equipped with an injection nozzle in the region of the cylinder barrel. The injection nozzle of a cylinder is in this case oriented toward the cylinder head; in some cases toward the outlet valve of the cylinder. This measure is intended to assist and accelerate the evaporation of the fuel particles or fuel droplets and thus assist and accelerate the mixture formation. At the same time, the head and the closed outlet valve are cooled by way of fuel. It is also sought to realize advantages in terms of pollutant emissions. It is also possible for two injection nozzles to be provided, which may possibly interact with one another, whereby it is sought to further improve the mixture formation. U.S. Pat. No. 5,421,301 describes such an internal combustion engine.
An injection nozzle which is oriented toward the cylinder head—as described in U.S. Pat. No. 5,421,301—supplies fuel only to the cylinder-head-side region of the combustion chamber during the course of the injection process, whereas the region of the combustion chamber between the injection device and bottom dead center, that is to say the piston-side region of the combustion chamber, remains disregarded during the injection.
The arrangement of the injection device in the region of the cylinder barrel yields further disadvantages. For example, an injection device provided in the region of the cylinder barrel is received by a recess or bore in the cylinder barrel, wherein the injection device is generally arranged in a sunken position such that a dead volume, in which injected fuel can and does collect, forms between the combustion-chamber-side tip of the injection device and the virtual inner envelope surface of the cylinder barrel, which also approximately constitutes the running surface of the piston. Fuel and coking residues resulting from incomplete combustion of fuel in the presence of a deficiency of oxygen are deposited on the injection device.
The deposition of liquid fuel on the injection device leads to increased untreated emissions of unburned hydrocarbons. The deposits on the injection device also lead to increased particle emissions of the internal combustion engine. This is because injected fuel accumulates in the porous coking residues, which fuel, often toward the end of the combustion when the oxygen provided for the combustion has been almost completely consumed, undergoes incomplete combustion and forms soot, which contributes to the increase in untreated particle emissions of the internal combustion engine.
Coking residues may also become detached from the injection device for example as a result of mechanical loading caused by a pressure wave propagating in the combustion chamber or the action of the injection jet. The residues detached in this way not only increase the untreated particle emissions of the internal combustion engine but may also lead to damage, and for example impair the functional capability of exhaust-gas after treatment systems provided in the exhaust-gas discharge system.
Furthermore, the coking residues can change the geometry of the injection device, in particular can adversely affect the through flow characteristic and/or impede the formation of the injection jet, and thereby disrupt the mixture preparation.
Additional concepts may be intended to counteract the build-up of coking residues and/or which serve to deplete deposits of coking residues, that is to say to remove said coking residues from and clean the combustion chamber. The German laid-open specification DE 10 199 45 813 A1 describes a concept of said type. Measures proposed for cleaning the combustion chamber include the targeted initiation of knocking combustion and/or the introduction of a cleaning fluid into the intake combustion air. Both measures may be regarded as relevant with regard to fuel consumption and pollutant emissions. The European patent EP 1 404 955 B1 describes an internal combustion engine whose at least one combustion chamber has, at least in regions, a catalytic coating on the surface for the purposes of oxidation of coking residues.
The inventors herein have recognized issues with the above approaches. For example, the concepts described above generally relate to an injection nozzle arranged in the cylinder head and projecting into the combustion chamber, and are not suitable for injection devices arranged in sunken fashion in the region of the cylinder barrel, that is to say for the removal or reduction of fuel or coking residues situated in a depression, that is to say in the dead volume.
Accordingly, an example approach is provided herein to at least partly address the above issues. In one example, a direct-injection internal combustion engine includes a cylinder head with a cylinder, the cylinder having at least one inlet opening for supply of combustion air via an intake system and at least one outlet opening for discharge of the exhaust gases via an exhaust-gas discharge system, the cylinder further comprising a combustion chamber which is jointly formed by a piston crown of a piston, by a cylinder barrel which laterally delimits the combustion chamber, and by the cylinder head, the piston being movable along a piston longitudinal axis between a bottom dead center and a top dead center. The engine includes an injection device positioned in the cylinder barrel for direct introduction of fuel into the combustion chamber, which injection device has at least one opening which, during a course of an injection process, is configured to be activated to introduce fuel into the combustion chamber, the injection device terminating flush, at a combustion chamber side, with the cylinder barrel.
In this way, the fuel injection device is mounted in the cylinder barrel (e.g., cylinder liner) and has a combustion-chamber-side front face that is flush with the inner surface of the cylinder liner, without forming a dead volume. In this way, the risk of fuel deposits and coking residues on the injection device is eliminated or reduced.
In the internal combustion engine according to the disclosure, the injection device of a cylinder terminates flush, at the combustion chamber side, with the cylinder-specific cylinder barrel. In the context of the present disclosure, this means firstly that at least the combustion-chamber-side tip of the injection device contacts a virtual inner envelope surface of the cylinder barrel, which approximately forms the running surface of the piston. In this way, the dead volume that forms between the combustion-chamber-side end of the injection device and the envelope surface of the cylinder barrel is reduced, and in some cases eliminated entirely.
Along with the dead volume, the risk of fuel deposition on the injection device or on the tip thereof and the risk of formation of coking residues are also reduced.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.